A foamed styrene resin material is generally produced by using foamable styrene polymer particles as a raw material, foaming the foamable styrene polymer particles with heating using steam, etc. to form pre-foamed particles once, filling a closed mold having a lot of small pores with the pre-foamed particles, foaming again with heating using pressurized steam to fill voids between the pre-foamed particles and to fuse the pre-foamed particles each other, followed by cooling and further removal from the mold.
The above foamable styrene polymer particles are normally produced by suspending a styrene monomer in water to polymerize the styrene monomer and impregnating the resulting polymer with a foaming agent, or produced by suspending styrene polymer particles in water, feeding a styrene monomer continuously or intermittently to polymerize the styrene monomer and impregnating the resulting polymer with a foaming agent (seed polymerization method) as shown in Japanese Patent Publication No. 49-2994.
The above foamed styrene resin material is, for example, used in a wall material of buildings by interposing it into panels as a heat insulating material. Accordingly, since a high heat insulating property is required to the foamed styrene resin material, the thermal conductivity must be reduced as possible. As the foamed styrene resin material used as the heat insulating material, a foamed material having a expansion ratio of about 30 to 40 has hitherto been used, mainly.
On the other hand, in order to reduce the production cost in the foamed styrene resin material, it is necessary to reduce the amount of styrene used as a raw material or reduce the thickness of the foamed material. Also, there is such an advantage that the living space can be widen when the thickness of the foamed material is reduced. Therefore, it is required to obtain a foamed styrene resin material of a higher expansion ratio (e.g. expansion ratio of about 50 to 125) without deteriorating the heat insulating property.
As a method of obtaining the foamed styrene resin material of the styrene resin, Japanese Patent Publication No. 57-34296 discloses that a specific thiourea compound is contained in styrene polymer particles, together with a foaming agent, to obtain a foamed material in which a lot of fine cells are formed. Furthermore, Japanese Patent Publication No. 55-49631 discloses that a specific thiodipropionate ester or thiodibutyrate ester is contained in styrene polymer particles, together with a predetermined foaming agent, to obtain a foamed material in which a lot of fine cells are formed like the one described above.
However, the foamed styrene resin material has a feature that the thermal conductivity becomes high when the expansion ratio becomes high. For example, according to the graph illustrating a relation between the specific gravity and the thermal conductivity shown in "1. Thermal Conductivity" in "6-2 Physical Properties of Foam (for general-purpose) using Polystyrene Particles" described in page 89 of Shuchi-Kanyo Gijutsu-shu 57 (1982)-133[3347] issued on Aug. 3, 1982 by the Japanese Patent Office, it is disclosed that the thermal conductivity is about 0.030 kcal/m.multidot.h.multidot..degree. C. when the expansion ratio is 33 (specific gravity: 30 g/l) while the thermal conductivity increases to about 0.034 to 0.035 kcal/m.multidot.h.multidot..degree. C. when the expansion ratio is 50 (specific gravity: 20 g/l).
The same is also described in Japanese Patent Laid-Open Publication No. 56-50935. That is, the gazette discloses that, in the foamed material of a synthetic resin such as polystyrene, etc., the thermal conductivity becomes minimum when the expansion ratio is from 20 to 30 and the thermal conductivity increases with the increase of the expansion ratio. Japanese Patent Laid-Open Publication No. 56-50935 discloses that an additive having a chemical structure which shows absorption to a specific infrared wavelength and a specific absorptance to black-body radiation at 300.degree. K. is contained in the foamed styrene resin material on the basis of a knowledge that such an increase in thermal conductivity at a high expansion ratio can be eliminated by reducing the influence of the radiation thermal conductivity.
However, the formulation of the above additive is likely to raise the cost and to exert a bad influence on the polymerizing and foaming steps.
Generally, the thermal conductivity of the foamed synthetic resin material is classified into (a) conduction of a solid phase, (b) conduction of a gas phase, (c) radiation between cell membranes and (d) convection of gas in cells on the basis of its conduction mechanism. In case of the high expansion ratio foam, since the volume occupied by the resin upon high expansion ratio is very small, the proportion occupied by the conduction of the solid phase (resin) of (a) in the thermal conductivity is small. The conduction of the gas phase of (b) is advantageous for reducing the thermal conductivity in case of using a Flon gas having a high molecular weight as the foaming agent. However, the influence exerted on the thermal conductivity decreases with a lapse of time because a gas is gradually released from the foamed material to be substituted with air. The convection of gas in cells of (d) is recognized when the cell diameter is not less than 4 mm, and it can be neglected in case of a normal foamed resin material. Accordingly, it is the radiation between the cell membranes of (c) that exerts the largest influence on the thermal conductivity.
The term "radiation" used herein means heat transfer which arises between two opposite surfaces having a different surface temperature. In the foamed material, the effect of damping the radiation heat transfer by the solid surface (resin) constituting the cells is large. Accordingly, it is considered that the cell diameter of the foamed material has a close relation to the isolation of the radiation heat, that is, the smaller the cell diameter is, the larger the number of heat flow isolation times per unit thickness (i.e. the number of cell membranes for damping the radiation heat) is, thereby reducing the thermal conductivity.
However, according to the study results of the present inventors, the thermal conductivity was not reduced in the foamed styrene resin material (high expansion ratio) having a density of not more than 0.02 g/cm.sup.3 even if the cell diameter becomes small.
A main object of the present invention is to provide a foamed styrene resin material which has a low thermal conductivity and excellent heat insulating property in spite of a low density (high expansion ratio foam).